LEDs are becoming popular for the lighting industry, particularly for backlighting the liquid crystal displays (LCDs.). The advantages of using LEDs for lighting equipment includes power saving, smaller size and no use of hazardous materials compared to fluorescent lighting devices. In addition, the power supply for LEDs usually operates with relatively low voltage which avoids any high-voltage potential issues associated with power supply for fluorescent lamps. For example, a cold cathode fluorescent lamp may require more than a thousand Volts AC to start and operate, whereas a single LED only requires about 1 to 4 Volts DC to operate.
To provide sufficient brightness, a display system requires many LEDs to produce comparable brightness as generated by a single fluorescent lamp. The challenge of using LEDs for lighting system is to optimize the brightness perception of human being eyes, in addition to balancing current in the LEDs. Brightness of color and color perception to human eyes vary significantly. For example, human eyes strongly perceive yellow color as comparing to green color. Therefore, in applications such as a traffic light, the amount of power delivered for the yellow light is lower than the power delivered for the green light to reach approximately equal eye perception.
There are different configurations for the multiple LEDs used in the lighting system. LEDs can be connected in series, in parallel or in serial-parallel combinations.
FIGS. 1A and 1B depict power supply circuits, 10 and 20, respectively, for parallel LEDs. Parallel LEDs receive a common supply voltage line from a power supply circuit. Usually, current is regulated by either monitoring the total amount of current in all the LEDs or the current in a single LED. Due to variation in the voltage drop of an LED, each LED may not carry the same current and therefore, produces different amount of brightness. Uneven brightness affects the lifetime of the LEDs. FIG. 1C shows a modified power supply circuit 30 so that each output provides power for one LED. In this case the power supply is complex and expensive. Such configuration is limited to low power LED system that contains few LEDs.
FIG. 2A depicts a power supply circuit 40 for serial LEDs. Each LED may have 1.0 Volt to 4.0 Volts voltage drop when an adequate amount of current is flowing through. It is the current flow in LED determines the brightness of the LED. The voltage drop correspondingly, depends on the manufacture of the LED, and the voltage drop can vary significantly. Therefore, the serial configuration has the advantage of regulating the string LED current so that each LED emits approximately same amount of brightness. For single-string LEDs, regulating the current of LED string for the power supply circuit is more suitable than regulating the voltage across the LED string. Power supply for such applications involves converting power source to a regulated output by current-mode control. Such application is bounded for number of LEDs in series which constitutes the voltage across the entire LED string. Too high a voltage limits the benefit of low-cost semiconductor device in the power supply circuit. For example, for a 12.1″ LCD display uses 40 LEDs for illumination. The voltage at the output of the converter may reach 150 Volts. The cost of the semiconductor switches to produce this voltage is prohibitive for such applications.
FIG. 2B depicts a power supply 50 for serial-parallel connected LEDs. Many LEDs are divided into multiple strings to reduce the cost of the converter circuit so that inexpensive semiconductor switches can be used. This configuration has the advantage of serial connection to provide the same amount of current flowing through the LEDs in the same string. The challenge, however, is in balancing the current among the strings as discussed in parallel LED configuration. The problem can be solved by using multiple power supplies with each power supply providing power to one string of LEDs. For example, each string of LEDs is operated by a separate DC/DC converter. However, multiple power stages for providing power to LED strings is bulky, not cost effective and is complicated. Often, this configuration may require synchronization of all power supplies to avoid any beat-frequency noise in the system.